92 THE WOMBAT. 



The uses Portland cement is put to are numerous, but 

 in any case it is seldom used in its neat state. For the 

 engineer it is in constant demand for foundations, piers and 

 harbour works, in the shape of concrete. Concrete is com- 

 posed of an aggregate (such as broken stone), sand and cement 

 in varying proportion according to the class of work to be 

 executed. A very usual specification is three parts of 

 aggregate to two of sand and one of cement. This, when set, 

 makes an extremely hard and compact mass. The builder 

 more often uses the cement mixed with sand only, for laying 

 bricks or plastering walls in the proportion of two or three 

 parts of sand to one of cement. 



It is sometimes contended that there are instances in 

 which the addition of sand in small quantities improves the 

 cement. Such instances are however quite the exception, for 

 sand can but be regarded as a diluent, and the only reason 

 that can be urged for its use is on the score of economy by 

 the admixture of so much cheaper a material. 



These are the chief uses, and I should like to enumerate 

 a few of the abuses, for cement is often asked to do work it 

 was never intended for. One of the most frequent causes of 

 failure in making concrete is the use of unsuitable aggregates, 

 such as a soft sandstone, or a half decomposed basalt or stone 

 overgrown with vegetable matters. Another great trouble is 

 the sand. Sand for concrete or mortar should be entirely free 

 from both loam and clay. Do not take sand out of the road 

 or ditch, or a fine drift sand ; neither let it be round-grained 

 or soft. To obtain the best results use a coarse, sharp, angular 

 sand such as one gets from quartz rocks. The greatest enemy 

 to cement is loam, clay or dirt, and it is of the utmost im- 

 portance that the aggregates or sand should be washed 

 perfectly clean before being used. Too much importance 

 cannot be laid upon this point. A very common practice on 

 the mining fields in this colony is to use the so-called sand 

 or sludge from the mullock heaps. At the best of times this 

 sand is quite unsuitable, having been ground to a very fine 

 state by the stampers, and to use it direct from the heap 

 without washing is throwing good money away, for ordinary 

 lime would answer just as well. 



Again, in hot weather cement commences to set very 

 quickly, and to get first class work it is necessary to have it 

 in its allotted place as soon as possible after the water has 

 been added. This necessitates mixing a very small quantity 

 at a time, a practice workmen do not care about, and often a 

 larger quantity is gauged than can be used before the setting 

 action commences. The result is that more water is added, 

 to the detriment if not the total destruction of the setting: 



